In a direct spot welding method, one electrode is brought into pressure contact with an upper surface of a top plate, and another electrode vertically aligned with the one electrode is bright into pressure contact with a lower surface of a bottom plate. In cases where an enough space for installation of the electrode is not available below the bottom plate, a one-side spot welding method is preferably employed wherein one electrode and another electrode are disposed on one side of the two plates to be joined. A typical example of such one-side spot welding method is disclosed in Japanese Patent Application Laid-open Publication (JP-A) No. 2006-181621.
FIG. 8 hereof shows the fundamental principle of the conventional one-side resistance spot welding method shown in JP 2006-181621A. As shown in this figure, a top plate 102 is placed on a bottom plate 101, and one electrode 103 and another electrode 104 are brought into contact with an upper surface of the top plate 102 with a predetermined distance held between the two electrodes 103, 104. The one electrode 103 is forced against the top plate 102 under the control of a pressure control unit 105, and the other electrode 104 is forced against the top plate 102 under the control of another pressure control unit 106. While keeping this condition, a current control device 107 operates to cause electric current to flow from the one electrode 103 via the top and bottom plates 102, 101 to the other electrode 104 along a path indicated by the arrows (1), (2) and (3).
During that time, as the electric current flows from the top plate 102 to the bottom plate 101 as indicated by the arrow (1), a so-called resistance heat generation phenomenon occurs at the interface between the top plate 102 and the bottom plate 101 at a position located directly beneath the electrode 103, and the weld is made. As a result, a weld nugget 108 is formed. Similarly, as the electric current flows from the bottom plate 101 to the top plate 102 as indicated by the arrow (3), a resistance heat generation phenomenon occurs at the interfaced between the top plate 102 and the bottom plate 101 at a position directly beneath the electrode 104, and the weld is made with the result that a weld nugget 109 is formed.
During the one-side resistance welding process, part of the electric current flows from the electrode 103 through the top plate 102 to the electrode 104, as denoted by reference numeral 111. This electric current part 111 is called as non-effective shunt current. The greater the amount of non-effective shunt current 111, the smaller the amount of effective current will be and formation of the nuggets 108, 109 becomes insufficient.
It is therefore desirable to reduce the amount of non-effective shunt current that flows from one electrode to another electrode through only one plate with which the electrodes are brought into contact.